Boron chelate complexes

ABSTRACT

Boron chelate complexes of the general formula  
                 
are described, where 
     X is either —C(R 1 R 2 )— or —C(R 1 R 2 )—C(═O)—, in which    R 1 , R 2  independently of one another denote H, alkyl (with 1 to 5 C atoms), aryl, silyl or a polymer, and one of the alkyl radicals R 1  or R 2  may be bonded to a further chelatoborate radical, 
 
or X denotes 1,2-aryl with up to two substituents S in the positions 3 to 6  
                 
   in which S 1 , S 2  independently of one another denote alkyl (with 1 to 5 C atoms), fluorine or a polymer,    as well as M +  denotes Li + , Na + , K + , Rb + , Cs +  or [(R 3 R 4 R 5 R 6 )N] +  or H + ,    where R 3 , R 4 , R 5 , R 6  independently of one another denote H or alkyl with preferably 1 to 4 C atoms.

The present invention relates to boron chelate complexes, a process fortheir production as well as their use as electrolytes and as catalysts.

Mobile electronics devices require increasingly more efficient andrechargeable batteries to provide an independent power supply. Besidesnickel-cadmium and nickel-metal hydride batteries, speciallyrechargeable lithium batteries, which have a substantially higher energydensity compared to nickel batteries, are suitable for this purpose. Thesystems available on the market have a terminal voltage of ca. 3 V; thisrelatively high potential means that water-based electrolyte systemscannot be used in lithium batteries. Instead, non-aqueous, mostlyorganic electrolytes (i.e. solutions of a lithium salt in organicsolvents such as carbonates, ethers or esters) are used in liquidsystems.

In the currently predominating battery design—lithium ion batteries withliquid electrolytes—practically exclusively lithium hexafluorophosphate(LiPF₆) is used as conducting salt. This salt satisfies the necessarypreconditions for use in high-energy cells, i.e. it is readily solublein aprotic solvents, leads to electrolytes with high conductivities, andhas a high level of electrochemical stability. Oxidative decompositionoccurs only at potential values greater than ca. 4.5 V. LiPF₆ hasserious disadvantages however, which can mainly be attributed to a lackof thermal stability (it decomposes above ca. 130° C.). Also, on contactwith moisture, corrosive and poisonous hydrogen fluoride is released,which on the one hand complicates handling and on the other hand attacksand damages other battery components, for example the cathode.

Given this background, intensive efforts have been made to developalternative conducting salts, and in particular lithium salts withperfluorinated organic radicals have been tested. These salts includelithium trifluoromethane sulfonate (“Li-triflate”), lithium imides(lithium-bis(perfluoroalkylsulfonyl)imides) as well as lithium methides(lithium-tris(perfluoroalkylsulfonyl)methides). All these salts requirerelatively complicated production processes and are therefore relativelyexpensive, and have other disadvantages such as corrosiveness withrespect to aluminium or poor conductivity.

Lithium organoborates have been investigated as a further class ofcompounds for use as conducting salt in rechargeable lithium batteries.On account of their low oxidation stability and safety considerations inthe handling of triorganoboranes, their use for commercial systems isexcluded however.

The lithium complex salts of the type ABL₂ (where A denotes lithium or aquaternary ammonium ion, B denotes boron and L denotes a bidentateligand that is bound via two oxygen atoms to the boron atom) proposed inEP 698301 for use in galvanic cells represent a considerable stepforward. The proposed salts, whose ligands contain at least one aromaticradical, however only have a sufficient electrochemical stability if thearomatic radical is substituted by electron-attracting radicals,typically fluorine, or if it contains at least one nitrogen atom in thering. Such chelate compounds are not commercially available and can beproduced only at high cost. The proposed products could therefore notpenetrate the market.

Very similar boron compounds are proposed in EP 907217 as constituentsin organic electrolytic cells. Compounds of the general formula LiBXX′are proposed as boron-containing conducting salt, where the ligands Xand X′ may be identical or different and each ligand contains anoxygen-containing electron-attracting group that binds to the boronatom. The listed compounds (lithium-boron disalicylate and a specialimide salt) exhibit the disadvantages already mentioned above however.

The compound lithium-bis(oxalato)borate (LOB) described for the firsttime in DE 19829030 is the first boron-centred complex salt describedfor use as an electrolyte that employs a dicarboxylic acid (in this caseoxalic acid) as chelate component. The compound is simple to produce,non-toxic and electrochemically stable up to about 4.5 V, which permitsits use in lithium ion batteries. A disadvantage however is that it canhardly be used in new battery systems with cell voltages of >3 V. Forsuch electrochemical storage units, salts with stabilities of ≧ca. 5 Vare required. A further disadvantage is that lithium-bis(oxalato)boratedoes not allow any possible structural variations without destroying thebasic chemical framework.

In EP 1035612 additives inter alia of the formulaLi⁺B⁻(OR¹)_(m)(OR²)_(p)are mentioned,with

-   m and p=0, 1, 2, 3 or 4, where m+p=4, and-   R¹ and R² are identical or different and are optionally directly    bonded to one another by a single or double bond, in each case    individually or jointly denote an aromatic or aliphatic carboxylic    acid or sulfonic acid, or in each case individually or jointly    denote an aromatic ring from the group consisting of phenyl,    naphthyl, anthracenyl or phenanthrenyl, which may be unsubstituted    or mono to tetra substituted by A or Hal, or in each case    individually or jointly denote a heterocyclic aromatic ring from the    group comprising pyridyl, pyrazyl or bipyridyl, which may be    unsubstituted or mono to tri substituted by A or Hal, or in each    case individually or jointly denote an aromatic hydroxy acid from    the group consisting of aromatic hydroxycarboxylic acids or aromatic    hydroxysulfonic acids, which may be unsubstituted or mono to tetra    substituted by A or Hal, and    -   Hal=F, Cl or Br, and    -   A=alkyl radical with 1 to 6 C atoms, which may be mono to tri        halogenated.

As particularly preferred additives there may be mentionedlithium-bis[1,2-benzenediolato(2-) O,O′]borate(1-),lithium-bis[3-fluoro-1,2-benzenediolato(2-)O,O′]borate(1-),lithium-bis[2,3-naphthalenediolato(2-)O,O′]borate(1-),lithium-bis[2,2′-biphenyldiolato(2-)O,O′]borate(1-),lithium-bis[salicylato(2-)O,O′]borate(1-),lithium-bis[2-olatobenzenesulfonato(2-)O,O′]borate(1-),lithium-bis[5-fluoro-2-olatobenzenesulfonato(2-)O,O′]borate(1-), lithiumphenolate and lithium-2,2-biphenolate. These are all symmetrical lithiumchelatoborates of the type Li[BL₂].

Lithium-bis(malonato)borate, which is said to have an electrochemicalwindow up to 5 V, has been described by C. Angell as being anelectrochemically particularly stable simple lithium-(chelato)boratecompound. The compound in question has the disadvantage however that itis practically insoluble in the usual battery solvents (e.g. only 0.08molar in propylene carbonate), which means that it can be dissolved andcharacterised only in DMSO and similar prohibitive battery solvents (WuXu and C. Austen Angell, Electrochem. Solid-State Lett. 4, E1-E4, 2001).

Chelatoborates may furthermore be present in protonated form (i.e.H[BL₂]) where L is a bidentate ligand that is bound to the boron atomvia two oxygen atoms. Such compounds have an extremely high acidicstrength and may therefore be used as so-called super acids in organicsynthesis being used as catalysts for cyclisations, aminations, etc. Forexample, hydrogen-bis(oxalato)borate has been proposed as a catalyst forthe production of tocopherol (U.S. Pat. No. 5,886,196). The disadvantageof this catalyst however is the relatively poor hydrolytic stability.

The object of the present invention is to obviate the disadvantages ofthe prior art and in particular to provide substances for conductingsalts that can be produced relatively simply and inexpensively fromcommercially available raw materials, that have a sufficient oxidationstability of at least 4.5 V, and that are readily soluble inconventionally used “battery solvents”. Furthermore the substancesshould be relatively resistant to decomposition by water or alcohols.

This object is achieved by “mixed” boron chelate complexes of thegeneral formula

where

-   X is either —C(R¹R²)— or —C(R¹R²)—C(═O)—, in which-   R¹, R² independently of one another denote H, alkyl (with 1 to 5 C    atoms), aryl, silyl or a polymer, and one of the alkyl radicals R¹    or R² may be bonded to a further chelatoborate radical,-   or X denotes 1,2-aryl with up to two substituents S in the positions    3 to 6-   in which S¹, S² independently of one another denote alkyl (with 1 to    5 C atoms), fluorine or a polymer,-   as well as M+ denotes Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ or [(R³R⁴R⁵R⁶)N]⁺ or    H⁺,    -   where R³, R⁴, R⁵, R⁶ independently of one another denote H or        alkyl with preferably 1 to 4 C atoms.

It has surprisingly been found that the aforelisted borates having twodifferent ligands, one of which is the oxalato radical, havesignificantly better solubilities than the symmetrical comparativecompounds.

The following compounds are preferred: hydrogen-(malonato,oxalato)borate(HMOB), hydrogen-(glycolato,oxalato)borate (HGOB),hydrogen-(lactato,oxalato)borate (HLOB),hydrogen-(oxalato,salicylato)borate (HOSB) andbis-[hydrogen-(oxalato,tartrato)borate] (BHOTB), as well as the lithium,caesium and tetraalkylammonium salts of the aforementioned acids.

The solubility of the mixed borates, which is a better comparison withthat of the symmetrical comparative compounds, is demonstrated in Table1 by the example of the lithium compounds: TABLE 1 Solubilities ofvarious lithium borate complex salts (in mol/kg) at room temperature LOBLLB LMB LOMB LLOB LOSB THF 1.90 <0.01 0.09 0.17 1.59 1.21 PC 0.88 <0.010.02 0.17 0.17 1.50 γ-BL 1.55 0.02 0.13 0.62 0.96 0.98 1,2-DME 1.30<0.01 0.003 0.20 0.93 0.43 Acetone 1.82 <0.01 0.03 0.43 1.38 0.51LOB = Lithium-bis(oxalato)borateLLB = Lithium-bis(lactato)borateLMB = Lithium-bis(malonato)borateLOMB = Lithium-(malonato, oxalato)borateLLOB = Lithium-(lactato, oxalato)borateLOSB = Lithium-(oxalato, salicylato)borateTHF = TetrahydrofuranPC = Propylene carbonateγ-BL = γ-Butyrolactone1,2-DME = 1,2-dimethoxyethane

It is clear that the compound LOB which is not according to theinvention has in most cases the best solubility. What is surprisinghowever compared to the other symmetrical compounds (LMB and LLB) is thesubstantially improved solubility of the mixed chelatoborates. In thesolvent propylene carbonate the mixed LOSB is even substantially moresoluble than LOB.

Table 2 shows the hydrolysisability of various chelatoborates. TABLE 2Degree of hydrolysis of 5% solutions in water after stirring for 2 hoursat room temperature LOB LMB LMOB Degree of hydrolysis (%) >50 5 15

The metal salts with mixed boron chelate anions according to theinvention can dissolve in relatively high concentrations in the typicalaprotic solvents such as carbonates, lactones and ethers used forhigh-performance batteries. Table 3 gives the measured conductivities atroom temperature: TABLE 3 Conductivities of non-aqueous electrolyteswith mixed chelatoborate salts in γ-BL, 1,2-DME and THF at roomtemperature γ-BL 1,2-DME THF Concn.¹⁾ Cond.²⁾ Concn.¹⁾ Cond.²⁾ Concn.¹⁾Cond.²⁾ LLOB 0.96 2.61 0.93 6.52 1.59 2.91 LSOB 0.43 4.17 1.21 3.17 LMOB0.54 3.59 0.17 0.41 LMB 0.13 1.65 0.009 0.01 LLB 0.02 0.01 insoluble 0insoluble 0 LOB 1.04 6.96¹⁾in mmol/g²⁾in mS/cm

It can be seen from Table 3 that solutions of the mixed borate saltshave conductivities of >2 mS/cm necessary for the operation of lithiumbatteries. In contrast to this, the solutions not according to theinvention of the symmetrically substituted salts LMB and LLB havesignificantly lower or practically zero conductivities.

The conductivities may be optimised corresponding to the prior art by,for example, combining at least one solvent having a high dielectricconstant (for example ethylene carbonate, propylene carbonate) with atleast one viscosity-reducing agent (for example dimethyl carbonate,butylacetate, 1,2-dimethoxyethane, 2-methyltetrahydrofuran).

Furthermore the salts with mixed chelatoborate anions exhibit thedesired high degree of electrochemical stability. For example, thecompound lithium-(lactato,oxalato)borate according to the invention hasa so-called “electrochemical window” of ca. 5 V, i.e. it is stable inthe range between 0 and ca. 5 V (Li/Li⁺=0), as shown in FIG. 1.

The boron chelate complexes described above can be fixed to polymercompounds by known techniques. Thus, it is possible to remove the acidichydrogen atoms in the α-position to the carbonyl groups by means ofsuitable bases and to add the carbanionic species obtained in this wayto functionalised (e.g. halogenated) polymers.

The boron chelate complexes according to the invention can be producedby reacting boric acid or boron oxide with oxalic acid and the otherchelate-forming agent, optionally in the presence of an oxidic metalsource (e.g. Li₂CO₃, NaOH, K₂O) or an ammonium salt, for exampleaccording to the following equations:H₃BO₃+C₂O₄H₂+L²→H[B(C₂O₄)L²]+3H₂Oor0.5B₂O₃+C₂O₄H₂.2H₂O+L²+LiOH.H₂O→Li[B(C₂O₄)L²]+5.5H₂Owhere L² denotes dicarboxylic acid (not oxalic acid), hydroxycarboxylicacid or salicylic acid (which may be at most di-substituted).

Preferably stoichiometric amounts of the starting substances are used,i.e. the molar ratio boron/oxalic acid/chelate-forming agent L²/optionaloxidic metal source or ammonium salt is about 1:1:1:1. Small deviationsfrom the theoretical stoichiometry (e.g. 10% above or below) arepossible without having a marked effect on the chelatoborate endproduct. Thus, if one of the ligands is present in excess, thecorresponding symmetrical end product will occur to a greater extent inthe reaction mixture. Thus for example if >1 mole of oxalic acid is usedper equivalent of boron-containing raw material, thenbis-(oxalato)borate will be formed in significant amounts. If thereaction is carried out in the presence of an oxidic lithium rawmaterial, LOB is formed, which, in combination with the mixedchelatoborates according to the invention, does not interfere withapplications as a battery electrolyte. If on the other hand an acid L²whose symmetrical chelato compound is sparingly soluble is employed inexcess, then the byproduct M[B(L²)₂] can easily be separated by a simpledissolution/filtration step. It is important to use ca. 2 moles ofchelate-forming agent per equivalent of boron component. If thechelate-forming agent is not used in a stoichiometric amount, unreactedboron component or undesirable 1:1-adduct (HO—B(C₂O₄) or HO—BL²) willremain. If more than 2 moles of chelate-forming agent are used, thenunreacted chelate-forming agent will remain, which has to be separatedin a complicated procedure.

The reaction according to the above chemical equations is preferablycarried out by suspending the raw material components in a mediumsuitable for the azeotropic removal of water (e.g. toluene, xylene,methylcyclohexane, perfluorinated hydrocarbons with more than 6 C atoms)and removing the water azeotropically in a known way.

It is also possible to carry out the synthesis in aqueous solution. Inthis case the components are added to water in an arbitrary sequence andare concentrated by evaporation while stirring, preferably under reducedpressure. After removing most of the water a solid reaction productforms which, depending on the specific product properties, is finallydried at temperatures between 100° and 180° C. and under reducedpressure (e.g. 10 mbar). Alcohols and other polar organic solvents apartfrom water, are also suitable as reaction media.

Finally, the product can also be produced without adding any solvent,i.e. the commercially available raw materials are mixed and then heatedby supplying heat and dehydrated preferably under reduced pressure.

The acids H[BC₂O₄L²] produced in this way are used in organic synthesisas super acid catalysts, e.g. for condensations, hydroaminations ordebenzylations. Lithium salts of the mixed chelatoborates are used aselectrolytes in electrolytic cells, preferably lithium batteries. Theammonium and caesium salts may be used in electrolytic double-layercapacitors.

The invention is described in more detail hereinafter with the aid ofthe following examples.

EXAMPLE 1 Production of lithium-(lactato,oxalato)borate (LLOB) by meansof azeotropic drying

100.9 g of a 72.0 aqueous lactic acid solution (801 mmoles), 49.59 g ofboric acid (802 mmoles) and 100.9 g of oxalic acid dihydrate (800mmoles) were suspended in 300 ml (ca. 270 g) of toluene in an inert(i.e. dried and filled with protective argon gas) 1 l capacitydouble-jacket reactor equipped with cooler, Dean-Stark water separator,stirrer and thermometer. A total of 30.87 g (418 mmoles) of pure lithiumcarbonate were then carefully added in portions from a measuring bulbwhile stirring thoroughly. This resulted in a vigorous formation of gasand foam. The solids agglomerated to form a viscous paste, which howevercould be suspended by stirring vigorously.

After the formation of foam had stopped the temperature of the heatingoil was raised to 130° C. within about 1 hour. The azeotropically formedwater was removed in portions. After a total of 10 hours' refluxing atotal of 96.7 g of water had been separated.

The reaction mixture was cooled to 40° C. and poured onto a glass fritand filtered. The colourless solids were washed twice with toluene andonce with pentane.

The finely powdered product was dried first of all at room temperatureand then at −100° C. on a rotary evaporator.

Yield: 150.8 g (=97% of theoretical)

Analysis: Actual Theoretical Li 5.05 5.16 B 4.9 5.16

δ¹¹B (DMF): 8.8 ppm; in addition a very small amount (<5%) of byproductswith boron shifts at 10.1 and 7.5 ppm.

Thermogravimetry (TGA): decomposition starts at ca. 270° C.

EXAMPLE 2 Production of lithium-(oxalato,salicylato)borate (LOSB) bymeans of azeotropic drying

49.59 g of boric acid, 100.9 g of oxalic acid dihydrate and 110.45 g ofsalicylic acid were suspended in 400 ml of xylene in the apparatusdescribed in Example 1, 30.9 g of lithium carbonate were added inportions, and the whole was then refluxed for 6 hours. During this time78 g of water were separated.

The reaction mixture was cooled to 50° C., filtered and the insolublecolourless solid was washed with xylene and then with hexane. Acolourless powder was obtained after drying at room temperature under anoil pump vacuum:

Yield: 183.5 g (95% of theoretical)

δ¹¹B (DMF) 5.5 ppm; (in addition impurities at 7.4 ppm (LOB, ca. 10%)and 3.8 (LSB, ca. 13%)

TGA: decomposition starts at ca. 210° C.

The crude product was purified by recrystallisation in THF/diethylether.

EXAMPLE 3 Production of hydrogen-(salicylato, oxalato) borate (HSOB) bytotal concentration by evaporation on a rotary evaporator

61.8 g of boric acid, 138.1 g of salicylic acid and 126.1 g of oxalicacid dihydrate (in each case 1 mole) were mixed in a 1 l capacity roundbottom flask and heated on a rotary evaporator at only slightly reducedpressure (900 mbar) at 110° to 115° C. After about 15 minutes thereaction mixture fully liquified and water began to distil off. After afurther 30 minutes the pressure was reduced further, whereupon themixture boiled vigorously. Towards the end of the separation of thewater, (after about 2 hours counting from the start of the reaction),the reaction matter solidified at a pressure of 20 to 30 mbar and an oilbath temperature of 125° C. into hard, in some cases coloured, lumps.Small amounts (a few g) of an uncoloured, needle-shaped sublimate, whichwas identified as salicylic acid, were also observed.

The reaction matter was cooled and ground by means of a pestle andmortar. The now white, powdery reaction material was again dried toconstant weight on a rotary evaporator at 115° to 125° C. and finally 10mbar (2 hours).

Yield: 216 g (92%) of almost uncoloured powder

The product was extremely soluble in propylene carbonate,γ-butyrolactone, 1,2-dimethyoxyethane, acetone and dimethylformamide.

δ¹¹B (1,2-DME): 5.5 ppm (main product)

-   -   7.6 ppm (HOB, ca. 15%)    -   3.5 ppm (hydrogen-bis(salicylato)borate, ca. 10%)

The crude product was purified by recrystallisation from acetone/MTBE.

1-8. (canceled)
 9. A boron chelate complex of the formula where X iseither —C(R¹R²— or —C(R¹R²)—C(═O)—, R¹ and R² are independently selectedfrom the

 group consisting of H, C₁-C₅ alkyl, aryl, silyl and a polymer, if R¹ orR² are alkyl they may be bonded to a chelatoborate radical, or X is a1,2-aryl with up to two substituents S in the positions 3 to 6

in which S¹ and S² are independently selected from the group consistingof a C₂ to C₅ alkyl, fluorine and a and polymer; wherein M+ is selectedfrom the group consisting of Li⁻, Na⁻, K⁺, Rb⁺, Cs⁺, [(R³R⁴R⁵R⁶)N]⁻ andH⁺, wherein R³, R⁴, R⁵, and R⁶ are independently H or alkyl.
 10. A boronchelate complex according to claim 1, wherein the boron chelate complexis selected from the group consisting of hydrogen-(malonato, oxalato)borate, hydrogen-(glycolato, oxalato) borate, hydrogen-(lactato,oxalato) borate, hydrogen-(oxalato, salicylato) borate,bis-[hydrogen-(oxalato, tartrato) borate, or a lithium, caesium ortetraalkylammonium salt of the boron chelate complex.
 11. A method forthe production of a boron chelate complex according to claim 1,comprising the steps of reacting at least one of boric acid or a boronoxide with oxalic acid and a chelate-forming agent H—O—X—(CO)—O—H. 12.The method of claim 3, further comprising adding an oxidic alkali metalsource or an ammonium salt.